A comparison of political violence by left-wing, right-wing, and Islamist extremists in the United States and the world

Of these three ideologies, most prior research has contrasted those committed to right-wing and left-wing causes. Much of this research suggests that compared to left-wing extremists, right-wing extremists may be more likely to engage in politically motivated violence. In comparison to left-wing supporters, right-wing individuals are more often characterized by closed-mindedness and dogmatism (9) and a heightened need for order, structure, and cognitive closure (5). Because such characteristics have been found to increase in-group bias and lead to greater out-group hostility (10), violence for a cause may be more likely among proponents of right-wing ideologies. In contrast, in comparison to their right-wing counterparts, left-wing individuals score higher on openness to new experiences, cognitive complexity, and tolerance of uncertainty (5). They are also less likely to support social dominance (11), which could lead to their overall lower likelihood to use violence against adversaries. In line with this reasoning, some studies have demonstrated an empathy gap between liberal and conservative individuals (12). Finally, according to various conceptualizations and operationalizations of right-wing authoritarianism (RWA; 13–15), aggressive tendencies constitute an inherent component of this construct, with people high in RWA being more hostile toward others who violate norms than those low in RWA. A recent meta-analysis supported this conclusion, revealing a positive relationship between right-wing ideology and aggressive attitudes and behaviors (16). However, the study did not focus solely on politicized contexts and included only milder forms of aggression.

Moreover, not all prior studies have supported the idea of ideological asymmetry. For instance, one study (17) found no difference between liberals and conservatives in the empathy they felt toward their political opponents. Further, although aggression is indeed included in classic conceptualizations of RWA, there have been suggestions that RWA is not necessarily a unidimensional construct and that relations between its subdimensions and social outcomes may be more complex (18). Thus, it is unclear to what extent right-wing authoritarians support or engage in nonnormative violence such as terrorism. At the same time, recent work on left-wing authoritarianism suggests that left-wing beliefs could also be related to support for aggression when aggressive acts are aimed at social hierarchy (19).

More generally, there is growing evidence suggesting that extremists representing different ideologies may have more in common than has been assumed (20). For example, extreme liberals and conservatives both represent the social world in a similar, simplistic way, which distinguishes both groups from more moderate individuals (21). In studies of behavior in conflict, one study (22) found that both right-wing and left-wing extremists used more negative and angrier language than moderates did. Finally, a strong inclination to defend one’s beliefs against worldview-violating groups and a low tolerance for such groups has been identified for both liberals and conservatives (23). Taken together, this research suggests that left-wing and right-wing extremists could be equally likely to use violence to pursue their ideological goals.

In short, the findings regarding differences between ideological groups in their tendency to use political violence are inconsistent and mostly indirect. Another important limitation comes from the fact that these studies have investigated predominantly mainstream samples and, in most cases, relied on attitudinal measures of aggressive tendencies. Research that compares perpetrators of real-world acts of political violence on behalf of different ideological causes is rare. It is thus unclear to what extent the current literature on political ideology generalizes to such extreme cases. Finally, extant research that does examine differences between right-wing and left-wing individuals has been limited mostly to Western samples and has generally omitted other ideological groups such as those representing Islamist extremism.

While there are examples of religious terrorism from a number of denominations, in recent years Islamist terrorism has far and away received the most research and policy attention (24, 25). It has been especially singled out for deadly attacks against military and government installations and also for the promotion of martyrdom through suicide attacks (26, 27). Moreover, while religiously charged motives for terrorist attacks were common in the past, a unique feature of their most recent form is that the religious aspect of the movement has become the dominant one (28–30). Some researchers (31–33) consider this to be one of the main reasons for the increasing lethality of terrorism in the past two decades.

Although claims for the greater lethality of religiously motivated terrorism are common, there have been few empirical tests of the argument. One of the rare studies that investigated this claim showed that Islamist terrorist organizations had significantly higher casualty rates than other types of terrorist organizations, controlling for a wide variety of rival explanations (34). However, this study also found that when affiliation with the al-Qaeda network was controlled for, Islamist groups were only slightly more likely than non-Islamist groups to commit high casualty terrorist attacks. While useful, this study was limited to 7 years, excluded domestic attacks, and was completed before the meteoric rise of several major Islamist groups since 2005, including ISIS, Boko Haram, and al-Shabaab.

Prior theorizing could suggest that Islamist extremists may be more similar in their willingness to use violence to right-wing than left-wing extremists. Islamist extremism seems to share with far-right extremism such key traits as fundamentalism, closed-mindedness, authoritarianism, and dogmatism (32, 35). In line with this reasoning, one study (35) referred to Islamist extremists as “devoted actors” and noted the rigidity and closed-mindedness of adherents. Another study (36) concluded that Muslim countries are markedly more authoritarian than non-Muslim countries, even controlling for a range of potentially influential factors. However, while theoretically plausible, direct evidence testing the hypothesis about similarity between Islamist and other ideological groups in their use of violence is missing.

The purpose of this research was to determine whether there are significant differences in the propensity to use violence to further a political cause linked to ideological commitment to right-wing, left-wing, and Islamist extremism. By including relevant cases representing violent acts committed by the adherents of each of these three ideologies, we were able to directly compare these groups. Extending past research in the area, we explored these issues using two very different databases. Study 1 extends prior individual-level work examining cases of politically motivated violent and nonviolent behavior by individuals radicalized in the United States while controlling for possible individual-level covariates. Study 2 assesses the lethality of attacks perpetrated in the name of different ideological causes using an incident-based dataset of terrorist attacks worldwide, accounting for relevant country-level covariates. Even though both datasets are publicly open and a few studies (37, 38) have used the variables that are of focal interest to the current paper as control variables, to our knowledge no published research directly investigated the specific question that we are focusing on. Datasets and code for both studies are available at https://osf.io/5rhwf/?view_only=None.

The goal of the present study was to examine the relationship between ideology and political violence. On the basis of existing research, we expected that perpetrators motivated by right-wing causes would be more likely to engage in violence than those representing left-wing causes. However, given that past research was scarce and mostly restricted to self-reported measures or less extreme forms of political aggression, the exact pattern of this relationship was uncertain. Moreover, there was suggestive evidence that extremists representing different ideologies could be in fact more similar than different. Finally, most past research has either focused on Western social contexts and compared the supporters of left-wing and right-wing causes or has examined only Islamist extremism. A lack of direct comparison between these three different ideologies was a gap that we aimed to address.

The findings from two studies, characterized by very different scopes and units of analysis, provide substantial support for conclusions about the relationship between ideology and the use of politically motivated violence. First, data on extremists in the United States showed that left-wing radicals were less likely to use violence than right-wing and Islamist radicals. Second, using worldwide data we found that in comparison to right-wing and Islamist groups, attacks motivated by left-wing groups were less deadly. These substantive conclusions were not affected by the inclusion of a set of control variables. Thus, the main findings appear to be robust across levels of analysis (i.e., individuals, groups) and geographical scope of the data.

Our results are in line with past research showing that conservative ideology—represented in our datasets by both right-wing and Islamist causes—is positively related to violent political behavior. These results support the view that left-wing and right-wing extremists are not equivalent when it comes to the use of violence (48; see also [49] for related findings on the victims of hate crimes in the United States). Whereas our findings are not inconsistent with the idea that individuals espousing different ideologies may feel equally negative toward worldview-threatening others (50), they suggest that the social consequences of extreme right-wing hostility may be more harmful than those caused by the far left (see [50] for a similar point).

At the same time, available data leave us agnostic as to the exact causal process between these variables. It could be that characteristics of individuals who are drawn to right-wing causes predispose them to greater aggressiveness. Such a hypothesis would be in line with past research on the psychological makeup of conservatives versus liberals (5). It is also possible that it is rather the characteristics of the cause that drive the use of violence. For example, it could be that right-wing causes are associated with stronger moral convictions or a greater sense of threat among their followers, which makes their adherents willing to use more extreme measures to attain their cause. Finally, the content of the political cause (e.g., hierarchy-enhancing vs. hierarchy-attenuating) may interact with individual characteristics (e.g., value of equality vs. dominance) and result in varying levels of violence. Future research could investigate the exact mechanisms behind the relationships identified in our data.

In terms of violent behavior, those supporting an Islamist ideology were significantly more violent than the left-wing perpetrators both in the United States and in the worldwide analysis. However, comparisons for Islamist and right-wing cases differed for the two samples. For the US sample, we found no significant difference in the propensity to use violence for those professing Islamist or right-wing ideologies. By contrast, for the worldwide sample, Islamist attacks produced significantly more fatalities than those produced by right-wing as well as left-wing perpetrators. One possible explanation for the discrepancy between the two studies is that right-wing perpetrators were much more common in the US sample than in the worldwide sample. The greater prevalence of right-wing extremism in the US sample than in the worldwide sample adds complexity to our overall conclusions as it could suggest that different mechanisms may account for whether a certain type of extremism occurs in a given sociopolitical context and its lethality once it appears.

The inclusion of all three types of extremism allowed us to see, to our knowledge for the first time, how they compare with regard to political violence. Future research could investigate in greater depth the psychological similarities and differences between Islamist extremists and the other two ideological groups. Past research noted that both right-wing and Islamist violent extremists are members of reactionary movements that frame their causes in terms of defending rigid, traditional hierarchies and often have a goal of establishing exclusive and homogeneous communities (51, 52). In further support of similarities between these two groups, one study (53) observed that a notable proportion of jihadist group recruits had educational backgrounds suggesting a motivation to seek order and reinforce rigid, traditional hierarchies. These features suggest that Islamist extremism is not orthogonal to the left–right distinction that is typically studied in the psychological literature but rather that it overlaps with the right-wing end of the ideological spectrum. Future research using more diverse samples could provide empirical verification of this idea. On the other hand, a more fine-grained analysis of different political causes within the broad, left–right ideological dimension could help identify the features of the causes that are more likely to be associated with violence.

Despite the fact that we are unable to speak to the mechanism accounting for the relationship that we found, we believe that the uniqueness of this study lies in the type of data that we used. Unlike most prior research examining the propensity for violence for different ideological categories within the moderate samples, our research looks at actual violent behaviors. As such, it extends the existing literature by providing insights into the extreme ends of ideological commitment. Further comparison between extreme and moderate actors representing different ideologies as well as different types of political commitment (e.g., epistemic commitment to extreme beliefs vs. behavioral commitment to violent actions) would help provide a more detailed picture of the relationship between ideology and aggression.

Nonetheless, both datasets examined include relatively large numbers of cases over nearly a half-century. We were able to control for a wide variety of variables associated in the past with ideological violence and found similar results from two studies with very different units of analysis. Interestingly, in both datasets we observed a decrease of left-wing extremism over time and an increase of right-wing and Islamist extremism. While controlling for the time of the violent act did not change the pattern of our results, it would be important for future research to understand the sources of these temporal dynamics.

It is important to acknowledge several limitations of our data. First, both databases suffer from the missing data problems that are a common feature of open-source studies of political extremism (54, 55). Important information may simply be unavailable in print and electronic open-source media. In addition, it is likely that the cases with the least missing data are those most prominently covered by news agencies. The question is how this problem could affect the conclusions of our research. One implication could be that both PIRUS and GTD are more likely to include cases that are more rather than less serious in terms of fatalities. Given our focus on differences in the use of violence across ideological categories, this characteristic would be most threatening to our results if the media treated newsworthiness differently across ideological categories. Given the attention generated by Islamist terrorism in recent years (56), we think the most likely possibility here is that our data are skewed by a tendency for the media to be more vigilant in reporting Islamist attacks compared to right-wing or left-wing attacks. Given that more-violent attacks are likely to have drawn attention regardless, it is possible that additional scrutiny for Islamist attacks would bias the results downward and make these cases appear overall less violent.

Second, while the GTD purports to include every terrorist attack around the world, PIRUS represents a sample of known political extremists in the United States. Although the PIRUS research team endeavors to include a random sample of available cases, PIRUS quite likely overrepresents recent cases when more complete open-source information has survived. This suggests that the US sample may be more likely to exclude nonviolent cases that are older since they were less likely to be recorded. However, this may actually strengthen our results for the US analysis because left-wing cases were more common in the 1970s while right-wing cases have been more common in later decades.

Finally, we limited our GTD analysis to persons killed and excluded those injured, which is also included in the GTD. By contrast, the PIRUS data include as violent all cases where there was evidence that the perpetrator planned to achieve a violent outcome, whether this resulted in fatalities or not. Future research could investigate ideological differences in the type of harm that is caused by extremists. For instance, the extent to which they target symbolic objects or cause material destruction could also differ between the groups, and these differences do not necessarily need to follow the pattern identified in our study. Although we present the analyses for the number of people injured in SI Appendix and our substantive conclusions remain similar when we combine both, we also explain why—due to the type of data and their coding—we treat fatalities as a more reliable variable than injuries.

Despite the limitations of our data, the finding that right-wing and Islamist cases are more violent than left-wing cases may have special relevance for the current time period. The surge in Islamist terrorism following the 9/11 attacks, driven by groups like al-Qaeda and ISIS, has been widely perceived as resulting in the increased deadliness of worldwide terrorism attacks. More recently, there has been growing evidence of a rising tide of populist-driven right-wing extremism in countries around the world (57, 58). Nearly 50 years ago, Jenkins (59) noted that “terrorists want a lot of people watching and a lot of people listening, and not a lot of people dead.” Three decades later, Jenkins (60) modified his original observation and made it more in keeping with our results from right-wing and Islamist extremists: “many of today’s terrorists want a lot of people watching and a lot of people dead.”

K.J.’s work was supported by a NAWA grant (number PPN/BEK/2019/1/00393/U/00001). APC was funded by a grant from the Strategic Program Excellence Initiative at the Jagiellonian University and ​with ​financial support ​from University of Maryland and The Pennsylvania State University.